1. Field of the Invention
The present invention relates to a method of manufacturing a heterojunction bipolar transistor and, more particularly, to a surface passivation technique for an outer periphery of an emitter-base junction or an outer periphery of a collector-base junction.
2. Related Background Art
A heterojunction bipolar transistor (HBT) has an ultra-high operation speed and a good current drive capacity compared to a field effect transistor, and thus attracts attention as a device of a new generation. An HBT directly uses an epitaxial crystal formed by MBE or MOCVD so that HBT characteristics are fully utilized. As a result, e.g., in an emitter-up HBT, etching is performed to form a base electrode, and accordingly the outer periphery of the emitter-base junction is exposed. Along with the micropatterning of an HBT, the emitter size of an emitter-up HBT or the collector size of a collector-up HBT is decreased. Then, surface recombination occurs in the exposed junction, posing a serious problem in decrease in gain. In other words, the higher the value (peripheral length)/(device area), the greater the influence from the surface.
As a method of solving this problem, it is conventionally proposed to form a base layer having an inclined structure. However, when the emitter or collector size becomes sufficiently small (e.g., 1 .mu.m.sup.2), this conventional method is not very effective. In addition, it is difficult to form a contact with the base having the inclined structure, resulting in numerous problems.
Moreover, in the above structure, dangling bonds still remain on the exposed surface of the HBT to disturb surface rearrangement. A surface level is then formed to serve as the carrier trapping center. In order to substantially solve this problem, it is important to perform passivation to decrease the dangling bonds.